This invention relates to a process for the production of a composition containing gold and/or silver particles, mixed oxides containing titanium and silicon which have been surface-modified, to the compositions producible in this process and to the use thereof in processes for the selective oxidation of hydrocarbons in the presence of oxygen and a reducing agent. The catalytically active compositions exhibit constantly high selectivities and productivities.
The direct oxidation of ethene to yield ethene oxide by molecular oxygen is well known and is used for the commercial production of ethene oxide in the gas phase. The typical catalyst for this application contains metallic or ionic silver, possibly additionally modified with various promoters and activators. Most such catalysts contain a porous, inert catalyst support having a small surface area, such as for example alpha-aluminium oxide, onto which the silver and promoters have been applied. A review of the direct oxidation of ethene in the presence of supported silver catalysts has been compiled by Sachtler et al. in Catalysis Reviews: Science and Engineering, 23 (1and2), 127-149 (1981).
It is also known that these silver catalysts and the reaction conditions which have proved favourable for ethene oxide production do not give rise to comparably good results in the direct oxidation of higher olefins, such as propene (U.S. Pat. Nos. 5,763,630, 5,703,254, 5,760,254) and propene oxide selectivities of at most approx. 50% are achieved. In general, direct oxidation reactions of these higher olefins with molecular oxygen do not generally proceed in the gas phase at below 200xc2x0 C., even in the presence of catalysts, and the selective production of oxidation products sensitive to oxidation, such as epoxides, is thus difficult as the consecutive reactions of these products frequently proceed more rapidly than the oxidation of the introduced olefins themselves. Another problem arises from the sensitivity to oxidation of the allyl groups present in higher olefins.
For this reason, only indirect, liquid phase methods are currently used for the industrial production of propene oxide.
Some 50% of propene oxide output worldwide is currently produced using the xe2x80x9cchlorohydrin processxe2x80x9d, while another 50%, with a rising trend, is produced by the xe2x80x9coxirane processxe2x80x9d.
In the chlorohydrin process (Weissermel et al. in Industrielle organische Chemie, 4th edition, Weinheim, 1994, pages 288-289), chlorohydrin is first formed by reacting propene with HOCl (water and chlorine) and the propene oxide is then formed therefrom by elimination of HCl with a base. The process is cost-intensive but, when properly optimised, exhibits high selectivity ( greater than 90%) with elevated conversions. The loss of chlorine in the chlorohydrin process in the form of worthless calcium chloride or sodium chloride solutions and the associated high waste water salt loads quickly led researchers to seek out chlorine-free oxidation systems.
The oxirane process (Weissermel et al. in Industrielle organische Chemie, 4th edition, Weinheim, 1994, pages 289-291), uses organic compounds to transfer oxygen onto propene instead of the inorganic oxidising agent HOCl. This indirect epoxidation is based on the fact that organic peroxides such as hydroperoxides in the liquid phase are capable of selectively transferring their peroxide oxygen onto olefins to form epoxides. This reaction converts the hydrogen peroxides into alcohols and the peroxycarboxylic acids into acids. Hydroperoxides are produced from the corresponding hydrocarbon by autoxidation with air or molecular oxygen. One serious disadvantage of indirect oxidation is the economic dependency of the value of propene oxide on the market value of the co-product and the cost-intensive production method for the oxidising agents.
There is currently no industrial gas phase process for the direct oxidation of propene to yield propene oxide.
Catalysts are known in which gold particles are applied onto a support consisting of dispersed titanium centres on a silicon matrix (WO 98 00415 A1; WO 98 00414 A1; EP 0 827 779 A1). All these materials obtained by impregnation with subsequent calcination become deactivated over time (typical half-lives are 5-50 h) and thus cannot be used in large scale industrial plants.
Further catalysts are also known in which gold particles are applied onto microporous, crystalline tectosilicates having a defined pore structure, in which silicon tetrahedron sites are isomorphically substituted by titanium (for example TS-1, TS-2, Ti zeolites, such as Ti-beta, Ti-ZSM-48 or mesoporous molecular sieves containing titanium, such as for example Ti-MCM-41 or Ti-HMS) (WO 9800413 A1). While all these gold silicalite or gold zeolite structures do indeed exhibit good selectivities, the hydrocarbon conversion rates and, most particularly, the catalyst service lives are completely inadequate for use in the chemicals industry.
The described processes for catalyst preparation are highly unsatisfactory with regard to catalyst activity and service life. Industrial processes operating with low activity catalysts require enormous reactors. Low catalyst service lives entail production downtime during the regeneration phase or demand cost-intensive redundant plant design. It is thus desirable to develop catalysts which can achieve elevated levels of activity combined with excellent selectivity and industrially useful service lives.
One object of the present invention was accordingly to provide a technically straightforward catalytic gas phase process for the selective oxidation of hydrocarbons using a gaseous oxidising agent on low cost solid catalysts, which process combines very high selectivities and industrial useful catalyst service lives with high yields and low costs.
A further object of the invention was to provide catalysts having better service lives.
A further object of the invention was to provide a process which yields catalysts having better service lives.
The objects are achieved according to the invention by a supported composition containing gold and/or silver particles, titanium oxide and a support containing silicon, characterised in that the surface of the composition bears groups selected from among alkylsilicon, arylsilicon, alkyl groups containing fluorine or aryl groups containing fluorine.